1. Field of the Invention
This invention relates in general to analog integrated circuits in telecommunication systems, and more particularly to a bias technique for operating point control in multistage analog integrated circuits.
2. Description of Related Art
Analog integrated circuits (IC), such as differential amplifiers, integrated mixers, and buffers, have been widely used in telecommunication systems. One of the desirable features is to operate the parameters of the circuit, such as an average output voltage level and an input stage transconductance, over widely varying process parameters, supply voltages, and temperatures.
In existing multistage analog ICs, bias conditions of all stages are generally set by one current source. This current source controls an input stage transconductance (GM). This current source also controls a quiescent output voltage, such as an output common mode voltage (VOCM) at the output stage of the circuit. Accordingly, any change in the current source for the purpose of affecting an input stage transconductance (GM), for example, increasing GM to improve the performance of the circuit, also affects an average output voltage level, such as an output common mode voltage (VOCM). This is an undesirable feature in many cases, especially since large changes in the current source are usually required to change an input stage transconductance (GM) due to a square root function between GM and I (GM=SQRT(I*Mu*Cox*W/L), where Mu is mobility, Cox is gate capacitance, and W/L is the geometry of a transistor, for example, M1 as described below in FIG. 2), whereas an output common mode voltage (VOCM) is determined by a linear function between VOCM and I (VOCM=VDD−(I*R)/2).
A typical analog integrated circuit (IC) is shown in FIG. 1 which has an input stage, an intermediate stage, and a load stage. An exemplary implementation having a cascoded differential amplifier with resistive loads is shown in FIG. 2. The term “cascoded” is different from the term “cascaded”. The term “cascoded” is generally referred to as the arrangement of several components of a single device being connected to in a series of stages, one on top of another, for example an input stage, an intermediate stage, and an output stage, etc. The term “cascaded” is generally referred to as the arrangement of two or more devices being connected in series, one after another.
FIG. 2 illustrates an exemplary differential amplifier having an input stage, an intermediate stage, and an output stage. At the input stage, a differential input pair of transistors M1–M2 and current mirror transistors M3–M4 form an input stage transconductance. The cascodes, transistors M5–M6, form a current buffer at an intermediate stage. Resistors R1–R2 form a load at an output stage.
As shown in FIG. 2, the bias conditions of all three stages are set by one current source I1, including an input stage transconductance GM (GM=SQRT(I1*Mu*Cox*W1/L1) and a quiescent output voltage VOCM (VOCM=VDD−(I1*R1)/2), wherein VDD is a voltage supply, Mu is the mobility, Cox is a gate capacitance, and W1/L1 is a geometry of a transistor M1. Any changes in I1 for the purpose of affecting the input stage transconductance GM also affect the quiescent output voltage VOCM. This is an undesirable feature in many cases, especially since large changes in I1 are required to change GM due to the square root function between GM and I1, thereby causing much larger changes in VOCM due to the linear function between VOCM and I1.
It is with respect to these and other considerations that the present invention has been made.